Abstract
Bauxite residue is typically alkaline, has high sodium content and elevated concentrations of trace elements. Effective rehabilitation strategies are needed to mitigate potential environmental risks from its disposal and storage. Increasingly, the importance of viable soil faunal populations as well as establishment of vegetation covers is recognized as key components of successful rehabilitation. Inoculation with earthworms is a strategy for accelerating mine site rehabilitation, but little is known on the effects of bauxite residue properties on earthworm survival and viability. In the current study, earthworms (Eisenia fetida) were exposed for 28days to a series of bauxite residue/soil treatments (0, 10, 25, 35, 50, 75 and 100% residue) to evaluate possible toxic effects on earthworms, investigate the bioavailability of relevant elements (e.g. As, Cr, V), and assess the risk of element transfer. Results showed that soil containing ≥25% residue (pH≥9.8; ESP≥18.5%; extractable Na≥1122mg/kg) significantly impacted survival (mortality ≥28%) and reproduction (cocoon production inhibition ≥76%) of the exposed earthworms. Alkalinity, sodicity and bioavailable Na were identified as major factors causing toxicity and some earthworms were observed to adopt compensative response (i.e. swollen body) to cope with osmotic stress. Conversely, soil containing 10% residue (pH=9.1; ESP=9.2%; extractable Na=472mg/kg) did not elicit significant toxicity at the organism level, but biomarker analysis (i.e. superoxide dismutase and catalase) in earthworm coelomocytes showed an oxidative stress. Furthermore, earthworms exposed to soil containing ≥10% residue took up and accumulated elevated concentrations of Al, As, Cr and V in comparison to the control earthworms. We concluded that earthworm inoculation could be used in future rehabilitation programmes once the key parameters responsible for toxicity are lowered below specific target values (i.e. pH=9.1, ESP=18.5%, extractable Na=1122mg/kg for Eisenia fetida). Nonetheless, trace element uptake in earthworms should be regularly monitored and the risk to the food chain further investigated.
Highlights
About 120 million tonnes of bauxite residue are produced annually, increasing the global inventory of approximately 2.7 billion tonnes of bauxite residue already disposed in land-based bauxite residue disposal areas (BRDAs) (Klauber et al, 2011)
The 100% fresh unrehabilitated bauxite residue (Fr) treatment was characterized by high alkalinity, moderate salinity (1.2 mS/cm) and high sodicity (41.5%)
These chemical properties decreased gradually as the residue was diluted with soil, showing the lowest values in the 10% Fr treatment
Summary
About 120 million tonnes of bauxite residue are produced annually, increasing the global inventory of approximately 2.7 billion tonnes of bauxite residue already disposed in land-based bauxite residue disposal areas (BRDAs) (Klauber et al, 2011). Rehabilitation of bauxite residue disposal areas (BRDAs) is essential to prevent pollution and risk to the wider environment (Directive 2006/21/EC). Research and industrial efforts have been mainly focusing on the establishment of a vegetation cover (i.e. revegetation) on the bauxite residue (Mendez and Maier, 2016; Wehr et al, 2006; Jefferson, 2004). Traditional criteria to assess the success of the rehabilitation have been mainly focused on vegetation establishment (Gautam et al, 2018; Courtney et al, 2009), overlooking soil communities, which are less visible and accessible (Lythe et al, 2017; Courtney et al, 2014; Majer et al, 2007). In this challenging frame of ecological restoration, earthworms are key players (Boyer and Wratten, 2010)
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